ENVE 201 Environmental Engineering Chemistry 1

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ENVE 201Environmental Engineering Chemistry 1

• CHLORINATION
• Dr. Aslihan Kerç

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Chlorination

• Disinfection of public water supplies and
  wastewater effluents.
• To prevent spread of water borne diseases (?)
• Cholara, typhoid ? by contamination of drinking
  water with wastewater
• Chlorination forms THMs

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• Alternative disinfectants
• Chlorine dioxide
• Ozone
• Emergency chlorination w/hypochlorites (1850)
• Continuous chlorination of public water supplies
• ? 1904 ( Calcium Hypochloride)

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• Calcium Hypochloride ?instable during storage
  ?limited usage
• Development of gaseous chlorine feeding
  facilities ?increased use
• Continual decline of waterborne disease

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• Current increase in waterborne diseases
• Giardiasis
• Cryptosporidium
• Infectious Hepatisis ( viral infection )

Protozoa
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Chlorine Chemistry

• Chlorine compound used in disinfection
• Chlorine gas Cl2
• Calcium Hypochlorite Ca(OCl)2
• Sodium hypochlorite NaOCl
• Chlorine dioxide ClO2
• (Cl- is not a disinfectant)

For small applications
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• Cl2 when applied to water ? forms hypochlorous
  acid and hydrochloric acid
• Cl2 H2O ? HOCl H Cl- (1)
• Stability constant for this rxn
• K HOClH Cl- / Cl2 4.510 -4 _at_25
  C

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• Ionization
• HOCl ? H OCl- (2)
• K H OCl- / HOCl 2.9 10-8 _at_ 25 C
• Free available chlorine HOCl OCl-

Variable w / temperature
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• Distribution between these species is important
• Killing effiency of HOCl is 40 -80 times larger
  than OCl - . Lower pH favors HOCl.
• HOCl Hypochlorous acid
• OCl - Hypochloride ion

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• Percentage distribution of HOCl and OCl -
• HOCl/ (HOCl OCl - 1 / ( 1 (OCl - /
  HOCl ))
• 1 / (1 (Ki/ H
• Hypochlorite salts
• Ca(OCl)2 2H2O ? 2HOCl Ca(OH)2
• NaOCl H2O ? HOClNaOH

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• Rxn(1) is dominated by Cl2. Obnoxious comp. NCl3
  may form ?requires high quality water
• For Chlorinator feed water ?use high quality
  water
• To avoid localized low pH ? flash mixing

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• Above pH 4 ? equilibrium (1) shifts to right.
• Cl2 decrease pH
• Hypochlorites increase pH
• Rxns. with impurities in water
• Cl2 and HOCl react with ammonia and humic
  material.

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• Rxns with ammonia
• Ammonium ion is in equilibrium with ammonia and
  hydrogen ion.
• NH4 ? NH3 H
• NH3 react with Cl2 or HOCl (hypochlorous acid)
• Rxns are dependent on pH , temperature , contact
  time , and Cl2 / NH3 ratio

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• Dominant Species
• Monochloramine (NH2Cl) and Dichloramine (NHCl2) ?
  combined available chlorine
• Chlorine readily reacts with reducing agents.
• Fe2 , Mn 2 , H2S , organic matter Chlorine is
  reduced to Cl.
• H2S Cl2 ? 2HCl S

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• These substances increase chlorine demand.
• Cl2 Phenols ?Produce mono-, di-,
• Trichlorophenols ? produce taste , odor
• Cl2 also reacts with other halogens
• Br- HOCl? HOBr Cl-
• HOBr Hypobromous acid

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• Cl2 and HOBr reacts with humic substance
• ? Halogenated organics. THMs
• Suspected human carcinogens.
• Maximum contaminant level 100 µg/L ? 80
  µg/L
• Alternative disinfectants ?

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• Cl2 is the only disinfectant producing protective
  residual within the distribution systems.
• Factors important in disinfection
• Time to contact
• Concentration
• Kill a Cn t

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Generalized curve obtained during breakpoint
chlorination
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Break Point Chlorination
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Break Point Chlorination

• Cl2 / NH3 ratio 11 for the formation of mono ,
  dichloroamines.
• Further increase in mole ratio ? trichloramine,
  oxidation of part of ammonia to N2 or NO3-.
• These rxns. are completed at mole ratio 1.51
• Chloramine residuals maximum _at_11mol
• Then decline to a minimum till 1,51

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Breakpoint Chlorination

• Chlorination of a water to the extent that all
  the ammonia is converted to N2 or higher
  oxidation states.
• Theoretically
• 3 mole chlorine ? conversion to trichloramine
• 4 mole chlorine ?complete oxidation to nitrate
• 2NH3 3Cl2 ?N2 6H 6Cl-

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• Breakpoint chlorination ? for better
  disinfection, required to obtain free chlorine
  residual , if ammonia is present.
• Method of ammonia removal in ww
• Combined chlorine residuals ? Longer lasting (
  final treatment with ammonia )
• Chlorine demand Amount of chlorine that must be
  added to reach a desired level of residual.

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Chlorine Residual Determination

• Old Methods ? total chlorine
• New Methods ? free and combined chlorine
• Total Chlorine Residual
• Measurement depend on measuring the oxidizing
  power
• Other oxidizing agents present may interfere ?
  manganese, nitrites

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Starch Iodide Method

• Oxidizing power of free and combined chlorine to
  convert iodide to iodine.
• Cl2 2I- ?I2 2Cl-
• I2 starch ?blue color
• Blue color ? shows the presence of free chlorine.